1. Field of the Invention
The present invention relates generally to cleaning devices for printing presses and, more particularly, to a method and apparatus for removing foreign matter adhering to the surface of a printing plate mounted on a rotating plate cylinder.
2. Description of the Prior Art
Planographic printing presses typically incorporate an image-bearing printing plate which is mounted on a rotating plate cylinder. Ink is applied to the image-bearing portion of the printing plate and the ink is then transferred by suitable means to the paper on which the image is printed. Lithographic and offset printing presses operate in this manner.
A serious problem which has plagued the printing industry concerning printing presses of this type is the adherence of dirt and other particles of foreign matter to the surface of the printing plate. Such foreign matter may comprise, for example, airborne dust, agglomerated ink solids and paper fluff. Once these particles adhere to the surface of the printing plate they are covered with ink in the next inking operation and form a spot or other imperfection which is transferred to the printed sheet. Hence, these particles interfere with the faithful reproduction of the image borne by the printing plate.
These particles are known in the trade as "hickeys." Their occurrence not only increases the cost of printing, but also poses a safety threat to press operators. In this regard, to shut down a printing press to remove the hickeys is a very costly proposition. However, an unacceptable print job is even more costly. A well known method of removing hickeys is the "thumb" method. This involves an attempt by the press operator to "flick" the hickey off the surface of the printing plate while it is moving at high speed. Of course, this method is very dangerous and completely impractical and, unfortunately, has resulted in the loss of numerous thumbs.
Devices have been developed which purport to safely and effectively remove particles of foreign matter from the surface of printing plates. These include arrangements for movably supporting a scraping blade in close proximity to the rotating plate cylinder. When a defect in the finished product is noticed, the scraper is moved into contact with the surface of the printing plate to attempt to dislodge the adhering particles. Some of these devices incorporate narrow scrapers which must be moved to a position along the width of the plate cylinder which corresponds to the location of the defect, and hopefully the location of the foreign particles. Operation of a device of this type is a hit-and-miss proposition. Other devices incorporate elongated scrapers which contact the entire width of the printing plate when actuated. Another device which has been developed to remove these particles comprises a rubber or leather roller having an irregular surface in constant rolling contact with the plate cylinder. While these may be effective to remove particles for a short period of time, they must be periodically removed from the press and cleaned by hand, resulting in significant press down time of up to a full day for each cleaning.
These prior art devices are relatively costly to manufacture and incorporate into existing printing presses. They are either too abrasive to the printing plate or too soft to have any meaningful service life. Most importantly, they have not proven to be effective solutions to the hickey problem. Hence, very few printing presses in use today incorporate plate cleaning devices at all, resulting in the common but dangerous use of the "thumb method."